Current estimation circuit

ABSTRACT

A current flowing through a switching element ( 5 ) of a power supply is detected by an AC current transformer ( 8 ), and a capacitor ( 201 ) is charged by a voltage corresponding to the current. A reduction factor of the terminal voltage of the capacitor in an off period of the switching element ( 5 ) is calculated based on an amplification factor of the terminal voltage of the capacitor, an absolute value of an instantaneous value of an input power supply voltage, and an instantaneous value of a direct current output voltage, and the capacitor is discharged so that the terminal voltage of the capacitor decreases by the reduction factor in an off period of the switching element ( 5 ). A current flowing through an inductor ( 4 ) is estimated from the terminal voltage of the capacitor.

TECHNICAL FIELD

The present invention relates to a current estimation circuit that, in aswitching power supply device that converts an alternating current ordirect current input power supply voltage into a desired direct currentoutput voltage utilizing an energy accumulation and releasing action ofan inductor accompanying a turning on and off of a switching element,estimates a current flowing through the inductor.

BACKGROUND ART

Among switching power supply devices, there are those that detect acurrent flowing through an inductor, and control an input current oroutput current. The kind of power factor correction circuit, or thelike, that controls an alternating current input current into asinusoidal waveform and suppress harmonic currents to an alternatingcurrent power supply, shown in Patent Document 1, is known as oneexample thereof.

FIG. 10 is a circuit diagram showing a heretofore known example of apower factor correction circuit that has the same configuration as thepower factor correction circuit shown in Patent Document 1. In theheretofore known example, which is a boost chopper type power factorcorrection circuit, the output of an alternating current power supply 1is rectified by a full-wave rectifier 3, and the output voltage of therectifier 3 is applied to a MOSFET 5 via an inductor 4. The inductor 4accumulates and releases energy in accordance with a turning on and offof the MOSFET 5, and supplies the released energy to a smoothingcapacitor 7 via a diode 6. At this time, a voltage corresponding to thecurrent (inductor current) flowing through the inductor 4 is generatedat either end of a current detecting resistor 10.

Next, a description will be given of a control circuit 100 that controlsthe turning on and off of the MOSFET 5 in such a way that the powerfactor is corrected. The terminal voltage of the smoothing capacitor 7,that is, the direct current output voltage output from the outputterminals 2 a and 2 b, is divided by a voltage dividing circuit formedfrom resistors 103 and 104. Thereupon, a voltage error amplifier 105detects an error in the divided voltage with respect to a referencevoltage 106, and outputs an error signal indicating the error.

Meanwhile, the output voltage of the rectifier 3, which is a positivevoltage, is divided by a voltage dividing circuit formed from resistors101 and 102. A multiplier 107 executes a calculation whereby the dividedvoltage is multiplied by the above-mentioned error signal, and outputsthe result of the calculation as a current command. A current erroramplifier 108 detects an error in the inductor current with respect tothe current command, and outputs an error signal indicating the error.Thereupon, a PWM comparator 110 compares the error signal and a carriersignal 109, and outputs a duty ratio gate control signal correspondingto the value of the error signal.

The gate control signal is input into a gate of the MOSFET 5 via a gatedriver 111. Consequently, the on-off timing of the MOSFET 5 iscontrolled in such a way that the inductor current coincides with thecurrent command, as a result of which, the direct current output voltageis controlled so as to become the voltage specified by the referencevoltage 106, and an average value of the inductor current is controlledto be a sinusoidal waveform. Although a phase compensation component isprovided in each of the voltage error amplifier 105 and current erroramplifier 108, these are omitted from FIG. 10. Also, an invertingamplifier circuit for adjusting the polarity and size of a signal inputinto the inverting input terminal of the current error amplifier 108 isprovided between the current detecting resistor 10 and the invertinginput terminal of the current error amplifier 108, but this is alsoomitted.

The heretofore described kind of control is called an average currentcontrol, and is advantageous in that there is little distortion in thealternating current input current, even when a discontinuous mode, inwhich there exists a period in which the inductor current returns tozero every switching cycle, and a continuous mode, in which the inductorcurrent does not return to zero every switching cycle, are mixed.

However, when the inductor current is detected by the current detectingresistor 10 as heretofore described, a problem occurs in that the largerthe capacity of the switching power supply device, the greater the powerloss of the current detecting resistor 10, and the conversion efficiencydecreases. As a countermeasure, it is possible to reduce the power lossby using a DCCT (DC current transformer) incorporating a Hall element,or the like, in place of the current detecting resistor 10, but as theDCCT is comparatively expensive, using it leads to an increase in thecost of the device.

FIG. 11 shows a heretofore known example wherein comparatively low costACCTs (AC current transformers) 8 and 8 a are used as current detectionmeans. In the heretofore known example, the current flowing through theMOSFET 5 is detected by the ACCT 8, the current flowing through thediode 6 is detected by the ACCT 8 a, and these currents are synthesizedin a current detector circuit 300 a. Consequently, a signalcorresponding to the current flowing through the inductor 4 is outputfrom the current detector circuit 300 a.

FIG. 12 shows an example of a configuration of the current detectorcircuit 300 a. The current detector circuit 300 a includes a voltagelimiter formed from Zener diodes 301 a and 302 a provided betweensecondary coils of the ACCT 8, a voltage limiter formed from Zenerdiodes 305 a and 306 a provided between secondary coils of the ACCT 8 a,diodes 303 a and 307 a that rectify the output signals of the ACCTs 8and 8 a respectively, and a resistor 304 a connected between a cathodeconnection point (signal synthesis point) of the diodes 303 a and 307 aand a grounding point, wherein a signal voltage corresponding to thecurrent flowing through the inductor 4 is output from the signalsynthesis point.

Meanwhile, in Patent Document 2, a technology is described whereby aninductor current of a DC/DC converter is estimated using an ACCT and acharging and discharging of a capacitor.

RELATED ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2007-209130 (FIG. 5)-   Patent Document 2: JP-A-2003-348830

OUTLINE OF THE INVENTION Problems that the Invention is to Solve

With the heretofore known example shown in FIG. 11, it is possible toreduce the power loss accompanying the current detection but, as the twoACCTs 8 and 8 a are used, problems such as an increase in the number ofparts and an increase in part mounting space occur. Furthermore, withthe heretofore known circuit, as a surge voltage occurring when theMOSFET 5 is turned off increases due to the effect of the ACCTs 8 and 8a primary side coil inductance and the inductance of wiring connectingthe ACCTs 8 and 8 a to each other, the switching loss of the MOSFET 5increases, and there is a danger that as a result, it will not bepossible to achieve an object of improving the conversion efficiency.

Meanwhile, the technology described in Patent Document 2 assumes thatthe inductance value of the inductor is constant. That is, under thiskind of assumption, it is possible to calculate the inductor current inan off period with a comparatively high accuracy.

However, there exist kinds of inductor such that the more the currentincreases, the more the inductance value decreases. For example,inductors such as one that uses a dust core as a core material exhibit atendency for the inductance value to decrease in accordance with anincrease in the current.

Also, the inductance value of the inductor varies in accordance with thedirect current amount (DC component), even though the variation range ofthe current flowing through the inductor is the same, that is, eventhough the (maximum value−minimum value) of the current is the same, andgenerally, the larger the direct current amount, the lower theinductance value.

With the technology described in Patent Document 2, as it is notpossible to respond to this kind of inductance value variation, it isdifficult to stably estimate the current flowing through the inductor.

The invention, having been contrived bearing this kind of situation inmind, has an object of providing a current estimation circuit that canachieve a reduction in cost, a miniaturization, and a suppression ofswitching loss, and estimate a current flowing through an inductor witha high degree of accuracy, despite a variation in the inductance valueof the inductor.

Means for Solving the Problems

The invention is a current estimation circuit, in a switching powersupply device that converts an alternating current or direct currentinput voltage into a direct current output voltage utilizing an energyaccumulation and releasing action of an inductor accompanying a turningon and off of a switching element, that estimates a current flowingthrough the inductor, wherein, in order to achieve the heretoforedescribed object, the circuit includes current detection means thatdetects a current flowing through the switching element, and outputs acorresponding signal voltage, a capacitor that is charged by a signalvoltage from the current detection means, means that calculates anamplification factor of a terminal voltage of the capacitor,instantaneous value detection means that detects an absolute value of aninstantaneous value of the input voltage and an instantaneous value ofthe direct current output voltage, means that calculates a reductionfactor of the terminal voltage of the capacitor in an off period of theswitching element, based on the amplification factor of the terminalvoltage of the capacitor, the absolute value of the instantaneous valueof the input voltage, and the instantaneous value of the direct currentoutput voltage, and discharging means that discharges the capacitor sothat the terminal voltage of the capacitor decreases in accordance withthe reduction factor in an off period of the switching element.According to this configuration, it is possible to estimate the currentflowing through the inductor from the terminal voltage of the capacitor.

The current detection means can include a current transformer. Then, forexample, an AC current transformer is used as the current transformer.

A discharge prevention circuit that prevents a discharge of current fromthe capacitor may be provided between the current detection means andthe capacitor. The discharge prevention circuit can include a diode thatprevents the discharge of current. Also, the discharge preventioncircuit may include a switching circuit that is turned on and off at atiming at which the switching element is turned on and off.

For example, a differentiating circuit is used as the means thatcalculates the amplification factor of the terminal voltage of thecapacitor. It is possible to provide the differentiating circuit with aconfiguration that uses the capacitor as a differential calculatingcomponent. The differentiating circuit can have a configuration whereinit has an operational amplifier to an inverting input terminal of whichone end of the capacitor is connected, a reference potential is inputinto a non-inverting input terminal of the operational amplifier, and aresistor is connected between the inverting input terminal and an outputterminal of the operational amplifier.

The current estimation circuit can further include voltage selectionmeans that selects whichever of the signal voltage from the currentdetection means and the terminal voltage of the capacitor is the highervoltage as a voltage that estimates the current flowing through theinductor. Also, the voltage selection means can also select whichever ofa corresponding reference voltage when the signal voltage from thecurrent detection means is zero and the terminal voltage of thecapacitor is the higher voltage as a voltage that estimates the currentflowing through the inductor.

The current estimation circuit can further include voltage selectionmeans that selects whichever of a corresponding reference voltage whenthe signal voltage from the current detection means is zero and theterminal voltage of the capacitor is the higher voltage as a voltagethat estimates the current flowing through the inductor.

Taking the amplification factor of the terminal voltage of the capacitorto be +di/dt, the absolute value of the instantaneous value of the inputvoltage to be vin, and the instantaneous value of the direct currentoutput voltage to be vo, the reduction factor of the terminal voltage ofthe capacitor may be obtained by calculating (+di/dt)·(vo−vin)/vin.

Advantage of the Invention

According to the invention, as it is possible to estimate the inductorcurrent in an off period of the switching element using the currentflowing through the switching element, there is no need for currentdetection means that detects the inductor current in the off period.Consequently, it is possible to achieve a reduction in cost and aminiaturization, and it is possible to reduce inductance of wiringrelating to the current detection means, thereby suppressing switchingloss caused by this wiring inductance.

Furthermore, according to the invention, when the amplification factorof the current flowing through the switching element varies inaccordance with a variation of the inductance value of the inductor, thecapacitor is discharged by a reduction factor commensurate with thevarying amplification factor, meaning that it is possible to accuratelyestimate the current flowing through the inductor, even when theinductance value varies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an example of a configuration of aswitching power supply device to which is applied a current estimationcircuit according to the invention.

FIG. 2 is a circuit diagram showing a specific example of a currentdetector circuit.

FIG. 3 is a circuit diagram showing a specific example of the currentestimation circuit.

FIG. 4 (A) shows a waveform diagram for illustrating an action of thecurrent estimation circuit of FIG. 3 in a continuous mode, while (B)shows a waveform diagram for illustrating an action of the same circuitin an intermittent mode.

FIG. 5 is a circuit diagram showing another example of a configurationof the current estimation circuit.

FIG. 6 is a circuit diagram showing another example of a configurationof a differentiating circuit.

FIG. 7 is a circuit diagram showing another example of a configurationof a discharge prevention circuit.

FIG. 8 is a circuit diagram showing another example of a configurationof the current estimation circuit.

FIG. 9 (A) shows a waveform diagram for illustrating an action of thecurrent estimation circuit of FIG. 8 in a continuous mode, while (B)shows a waveform diagram for illustrating an action of the same circuitin an intermittent mode.

FIG. 10 is a circuit diagram showing a first heretofore known example.

FIG. 11 is a circuit diagram showing a second heretofore known example.

FIG. 12 is a circuit diagram showing a specific example of the currentdetector circuit in the heretofore known example of FIG. 11.

MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a circuit diagram of a boost chopper type power factorcorrection circuit shown as an example of a configuration of a switchingpower supply device to which is applied a current estimation circuitaccording to the invention. In FIG. 1, the same numbers are given tocomponents the same as, or common to, components shown in FIG. 11.Hereafter, a description of the same or common components will beomitted.

In a heretofore known example shown in FIG. 11, a current flowingthrough a diode 6, that is, a current flowing through an inductor 4during an off period of a MOSFET 5, is detected by an ACCT 8 a, which isan AC current transformer. In an embodiment of the invention, however,as inductance in the off period is estimated by a current estimationcircuit 200, shown in FIG. 1, added to a control circuit 100, the ACCT 8a is omitted.

Firstly, referring to FIG. 2, a description will be given of a specificexample of a current detector circuit 300 connected to an ACCT 8. Thecurrent detector circuit 300 includes a voltage limiter formed fromZener diodes 301 and 302 connected in series between secondary coils ofthe ACCT 8, a diode 303 that rectifies an output signal of the ACCT 8,and a resistor 304 connected between a cathode of the diode 303 and agrounding point. A signal corresponding to a current flowing through theinductor 4 in an on period of the MOSFET 5 is output from the currentdetector circuit 300. The above-mentioned voltage limiter is provided inorder to make an exciting current that excites an iron core of the ACCT8 practically zero in an off period of the MOSFET 5.

Next, referring to FIG. 3, a description will be given of a specificexample of the current estimation circuit 200. The current estimationcircuit 200 includes a discharge prevention circuit 400 including anoperational amplifier 401 and diode 402, a capacitor 201 connectedbetween an output terminal of the discharge prevention circuit 400 and agrounding point, a differentiating circuit 500 that differentiates aterminal voltage of the capacitor 201, a sample and hold circuit 202that holds an output signal value of the differentiating circuit 500, amultiplier 203 that multiplies an output of the sample and hold circuit202 by an output of a divider 206, and a voltage controlled currentsource 204 controlled by an output of the multiplier 203. Herein, thedischarge prevention circuit 400 outputs a voltage equivalent to thatinput by functioning as a voltage follower, and performs a function ofpreventing the charge of the capacitor 201 from being discharged on thedischarge prevention circuit 400 and current detector circuit 300 side,that is, preventing the charge of the capacitor 201 from beingdischarged via the output terminal of the operational amplifier 401,using the diode 402.

Herein, when taking an absolute value of an instantaneous value of theinput voltage (the output voltage of a rectifier 3 in the embodiment ofFIG. 1) to be vin, an instantaneous value of a direct current outputvoltage output from terminals 2 a and 2 b to be vo, an inductance valueof the inductor 4 to be L, and a current amplification factor of acurrent flowing through the inductor 4 in a period in which the MOSFET 5is on to be (+di/dt), the current amplification factor is expressed asin the following Equation (1).+di/dt=vin/L  (1)

Also, when taking a reduction factor of a current flowing through theinductor 4 in a period in which the MOSFET 5 is off to be (−di/dt), thecurrent reduction factor is expressed as in the following Equation (2).−di/dt=(vo−vin)/L  (2)

When L is deleted from Equations (1) and (2), the following Equation (3)is obtained.−di/dt={(vo−vin)/vin}×(+di/dt)  (3)

As is clear from Equation (3), the current reduction factor (−di/dt) canbe obtained by multiplying the current amplification factor (+di/dt) bythe proportional factor (vo−vin)/vin.

Meanwhile, a divided voltage having a voltage value vin′ correspondingto the instantaneous value vin of the input voltage is output from avoltage divider circuit formed from a resistor 101 and resistor 102shown in FIG. 1, and also, a divided voltage having a voltage value vo′corresponding to the instantaneous value vo of the direct current outputvoltage is output from a voltage divider circuit formed from a resistor103 and resistor 104 shown in FIG. 1.

In the embodiment, values of the resistors 101 to 104 are set so thatthe divided voltage ratios of the two voltage divider circuits areequal. Consequently, the relationship of the following Equation (4) isestablished between the proportional factor (vo−vin) of Equation (3) andthe output voltage values vin′ and vo′ of the voltage divider circuits.(vo−vin)/vin=(vo′−vin′)/vin′  (4)

Meanwhile, an output voltage vis of the current detector circuit 300shown in FIG. 1 is input into the discharge prevention circuit 400 shownin FIG. 3. Consequently, the capacitor 201 of FIG. 3 is charged by avoltage having a waveform the same as the waveform of the output voltagevis of the current detector circuit 300 in a period in which the MOSFET5 is on. At this time, a peak value of the terminal voltage of thecapacitor 201 corresponds to an initial value of a current flowingthrough the inductor 4 when the MOSFET 5 is turned off.

The differentiating circuit 500 differentiates a terminal voltage valuevs of the capacitor 201, and outputs a signal indicating the currentamplification factor (+di/dt) of the current flowing through theinductor 4 in a period in which the MOSFET 5 is on. The sample and holdcircuit 202 reads the value of the current amplification factor (+di/dt)based on an output signal vpwm of a PWM comparator 110, shown in FIG. 1,that specifies a period for which the MOSFET 5 is on, and holds thevalue for a period for which the MOSFET 5 is off.

Meanwhile, a calculation (vo′−vin′) whereby the voltage value vin′corresponding to the instantaneous value yin is subtracted from thevoltage value vo′ corresponding to the instantaneous value vo isexecuted in a subtractor 205, and also, a calculation (vo′−vin′)/vin′whereby the result of the subtraction (vo′−vin′) is divided by thevoltage value vin′ is executed in the divider 206. As is clear fromEquation (4), the proportional factor (vo−vin)/vin is obtained from thecalculation in the divider 206.

The calculation of Equation (3), whereby the value of the currentamplification factor (+di/dt) held in the sample and hold circuit 202 ismultiplied by the proportional factor (vo−vin)/vin, is executed in themultiplier 203, by which the reduction factor (−di/dt) of the currentflowing through the inductor 4 in a period in which the MOSFET 5 is offis obtained.

The voltage controlled current source 204 discharges the capacitor 201in accordance with the reduction factor (−di/dt) in an off period of theMOSFET 5. Because of this, the reduction factor of the terminal voltagevalue vs of the capacitor 201 coincides with the reduction factor of thecurrent flowing through the inductor 4 in an off period of the MOSFET 5.

That is, the terminal voltage value vs of the capacitor 201 increases byan amplification factor in accordance with the current amplificationfactor (+di/dt) in an on period of the MOSFET 5, and decreases by areduction factor in accordance with the current reduction factor(−di/dt) in an off period of the MOSFET 5.

In this way, according to the embodiment, as the terminal voltage valuevs of the capacitor 201 corresponds to the current flowing through theinductor 4, it is possible to estimate the current flowing through theinductor 4 in an off period of the MOSFET 5 from the terminal voltagevalue vs of the capacitor 201, despite not using the ACCT 8 a shown inFIG. 11.

FIG. 4 (A) shows a waveform diagram for illustrating an action of thecurrent estimation circuit 200 in a continuous mode (a mode in which theinductor current does not return to zero every switching cycle), whileFIG. 4 (B) shows a waveform diagram for illustrating an action of thesame circuit 200 in an intermittent mode (a mode in which the inductorcurrent returns to zero every switching cycle).

In the diagrams, (a) exemplifies the waveform of the output signal vpwmof the PWM comparator 110 (refer to FIG. 1) that specifies an on periodand off period of the MOSFET 5, (b) the waveform (refer to the solidlines) of the terminal voltage of the capacitor 201 in an on period ofthe MOSFET 5 and the waveform (refer to the dotted lines) of theterminal voltage of the capacitor 201 in an off period of the MOSFET 5,and (c) the waveform of the current flowing through the inductor 4.

As is clear from FIG. 4, as the terminal voltage vs of the capacitor 201increases and decreases in the same pattern as an inductor current iL,the terminal voltage vs corresponds to the inductor current iL.Therefore, in the embodiment, the value of the inductor current iL isestimated from the value of the terminal voltage vs of the capacitor 201output from the current estimation circuit 200. The terminal voltage vsof the capacitor 201 corresponding to the inductor current iL is inputinto the error amplifier 108, as shown in FIG. 1.

FIG. 5 shows another example of a configuration of the currentestimation circuit 200. By using a differentiating circuit 500 a withthe configuration shown, the capacitor 201 shown in FIG. 3 is omittedfrom the current estimation circuit 200.

The differentiating circuit 500 a has a heretofore known configurationincluding an input capacitor 501, a feedback resistor 502, and anoperational amplifier 503, wherein the input capacitor 501 is alsoutilized as alternative means to the capacitor 201. One end of the inputcapacitor 501 is connected to an inverting input terminal of theoperational amplifier 503, a ground potential, which is a referencepotential, is input into a non-inverting input terminal of theoperational amplifier 503, and the feedback resistor 502 is connectedbetween the inverting input terminal and an output terminal of theoperational amplifier 503.

Herein, assuming that the output voltage vis of the current detectorcircuit 300 shown in FIG. 1 is rising, the capacitor 501 is charged viathe discharge prevention circuit 400 so that the terminal voltagebecomes vis (=vs). At this time, a charging current i of the capacitor501 flows along a path from the discharge prevention circuit 400,through the capacitor 501 and resistor 502, to the output terminal ofthe operational amplifier 503. Then, taking an electrostatic capacity ofthe capacitor 501 to be C₅₀₁, a relationship of (1/C₅₀₁) ∫idt=vs isestablished, so the charging current i of the capacitor 501 correspondsto a differential value of the voltage vis (=vs).

Meanwhile, as the inverting input terminal of the operational amplifier503 is placed at a ground potential (0 volts) by a virtual shortcircuit, when taking a resistance value of the feedback resistor 502 tobe R₅₀₂, an output voltage −i of the operational amplifier 503multiplied by R₅₀₂ is also a value proportional to the differentialvalue of the voltage vis (=vs).

The differentiating circuit 500 a acts in the way heretofore described.Then, as heretofore described, the inverting terminal of the operationalamplifier 503 is placed at a ground potential by a virtual shortcircuit. Therefore, according to the current estimation circuit 200, asthe charging and discharging action of the capacitor 501, which is acomponent of the differentiating circuit 500 a, is the same as that ofthe capacitor 201, the capacitor 501 also has the function of thecapacitor 201, meaning that it is possible to reduce the number ofcapacitors used.

A differentiating circuit with the kind of configuration shown in FIG. 6may be applied in place of the differentiating circuit 500 a. Thedifferentiating circuit 500 a has a heretofore known configurationwherein an input resistor 504 is connected in series to the capacitor501 of the differentiating circuit shown in FIG. 5, and a feedbackcapacitor 505 is connected in parallel to the feedback resistor 502 ofthe same circuit.

The differentiating circuit 500 provided in the current estimationcircuit 200 shown in FIG. 3 can also be given the kinds of configurationshown as examples in FIG. 5 and FIG. 6.

Meanwhile, with the discharge prevention circuit 400 shown in FIG. 3 andFIG. 5, when the capacity value of the capacitors 201 and 501 is low,the capacitors 201 and 501 may be charged or discharged at an on-offtiming of the MOSFET 5 (refer to FIG. 1) due to the effect of a jointcapacitance or reverse recovery characteristics of the diode 402, inwhich case, the output voltage vs of the current estimation circuit 200and the current flowing through the inductor 4 cease to correspondexactly.

In order to avoid the heretofore described kind of problem, thedischarge prevention circuit 400 shown in FIG. 7 is configured of avoltage follower circuit 401 a using an operational amplifier, and aswitching circuit 402 a connected to an output terminal of the voltagefollower circuit 401 a.

With this discharge prevention circuit 400, as the switching circuit 402a is controlled using the output signal vpwm of the PWM comparator 110(refer to FIG. 1), that is, as the switching circuit 402 a is turned onat a timing at which the MOSFET 5 is turned on, and the switchingcircuit 402 a is turned off at a timing at which the MOSFET 5 is turnedoff, the heretofore described problem, wherein the capacitor 201 ischarged or discharged due to the effect of the joint capacitance orreverse recovery characteristics of the diode 402, is avoided.

As the switching circuit 402 a, it is desirable to choose one with acircuit configuration such that the parasitic capacity thereof issmaller than the capacity of the capacitor 201 or capacitor 501, andfurthermore, the charge variation of the capacitor 201 or capacitor 501accompanying an on-off action thereof is small.

Meanwhile, with the control circuit 100 shown in FIG. 1, a positive andnegative power supply is normally used as a control power supply, but itis also possible to use a single power supply as the control powersupply in order to achieve a simplification of the power supplyconfiguration. When using a single power supply as the control powersupply, it is common that the control reference potential is a biasedvoltage.

FIG. 8 shows an example of a configuration of the current estimationcircuit 200 embedded in a control circuit using the heretofore describedsingle power supply. A discharge prevention circuit 400 b of the currentestimation circuit 200 includes an operational amplifier 401 b, aswitching circuit 402 b connected in series to an output terminal of theoperational amplifier 401 b, voltage dividing resistors 403 b and 404 b,one end of each of which is connected to a non-inverting input terminalof the operational amplifier 401 b, a resistor 405 b, one end of whichis connected to an inverting input terminal of the operational amplifier401 b, and a resistor 406 b connected between the inverting inputterminal and output terminal of the operational amplifier 401 b.

In the discharge prevention circuit 400 b, a bias voltage Vbias1 isapplied via the resistor 405 b, and a bias voltage Vbias2 (set to anyvalue larger than that of the bias voltage Vbias1) is applied via thevoltage dividing resistor 403 b. The voltage vis is input via thevoltage dividing resistor 404 b.

Meanwhile, although a differentiating circuit 500 b of the currentestimation circuit 200 has a configuration equivalent to that of thedifferentiating circuit 500 a shown in FIG. 5, the bias voltage Vbias1is applied as a reference potential to a non-inverting input terminal ofan operational amplifier 503 b. Also, a feedback resistor 502 bcorresponding to the feedback resistor 502 of FIG. 5 is connectedbetween an inverting input terminal and output terminal of theoperational amplifier 503 b.

The divided voltage ratios of the voltage dividing resistors 403 b and404 b are adjusted so that the voltage of the non-inverting inputterminal of the operational amplifier 401 b is Vbias1 when the outputvoltage vis of the current detector circuit 300 shown in FIG. 1 is zero,because of which, an output voltage visb of the operational amplifier401 b is zero when the signal value vis is zero.

A maximum value circuit 207 compares a terminal voltage vsb of acapacitor 501 b and the output voltage visb of the operational amplifier401 b, outputs vsb as the voltage vs indicating the inductor currentwhen vsb≧visb, and outputs visb as the voltage vs indicating theinductor current when vsb<visb.

FIG. 9(A) shows a waveform diagram for illustrating an action of thecurrent estimation circuit 200 shown in FIG. 8 in a continuous mode (amode in which the inductor current does not return to zero everyswitching cycle), while FIG. 9(B) shows a waveform diagram forillustrating an action of the same circuit 200 in an intermittent mode(a mode in which the inductor current returns to zero every switchingcycle). In FIG. 9, the reference characters vsb indicate the terminalvoltage of the capacitor 501 b. When the switching circuit 402 b isturned on, the voltage vsb naturally coincides with the output voltagevisb of the operational amplifier 401 b.

As shown in FIG. 9, the voltages vsb, visb, and vs vary with the biasvoltage Vbias1 as a reference potential.

Herein, an explanation will be given of the reason for providing themaximum value circuit 207. When using the discharge prevention circuit400 b including the switching circuit 402 b, the following kind ofcondition occurs. That is, as shown in the waveform diagram of FIG.9(B), the inductor current iL becomes zero in an off period of theMOSFET 5 in the intermittent mode. At this time, as the switchingcircuit 402 b is off, the terminal voltage vsb of the capacitor 501 bbecomes smaller than the voltage visb (the voltage visb when theswitching circuit 402 b is off is equivalent to the bias voltage Vbias1)on the input side of the switching circuit 402 b, as shown in (c) of thediagram.

When seeking to estimate the inductor current from the terminal voltagevsb, the inductor current is estimated to be an apparent negativecurrent in the heretofore described kind of condition. The maximum valuecircuit 207, as heretofore described, outputs visb as the voltage vsindicating the inductor current when vsb<visb. Consequently, byproviding the maximum value circuit 207, it is possible to avoid theheretofore described kind of problem wherein the inductor current isestimated to be a negative current.

The heretofore described configuration using a single power supply canalso be applied to the current estimation circuit 200 shown in FIG. 3and FIG. 5. In this case, it is possible to take the diode 402 to be aswitching circuit.

Also, as is clear from FIG. 9(A) and FIG. 9(B), it is also possible toreplace the bias voltage Vbias1 with the voltage visb as the referencevoltage when the voltage vis from the current detector circuit 300 iszero, and adopt it as one input signal of the maximum value circuit 207.

Also, in the embodiment, an alternating current voltage from analternating current power supply 1 is used as an input voltage of theinvention, and this, full-wave rectified, is used as an input into theswitching power supply device but, not being limited to this, a directcurrent voltage of a direct current power supply of a battery, or thelike, may also be used as the input voltage of the invention. In thiscase, the embodiment is a switching power supply device that is not apower factor correction circuit.

Furthermore, in the embodiment, a booster circuit is taken as an examplebut, the thinking behind the invention not being limited to this, theinvention can also be applied to a step-down circuit, a polarityreversing circuit, or the like.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   1 Alternating current power supply-   2 a, 2 b Direct current output terminal-   3 Full wave rectifier-   4 Inductor-   5 MOSFET-   6 Diode-   7 Smoothing capacitor-   8, 8 a ACCT-   100 Control circuit-   101, 102, 103, 104 Resistor-   105 Voltage error amplifier-   106 Reference voltage-   107 Multiplier-   108 Current error amplifier-   109 Carrier signal-   110 PWM comparator-   111 Gate driver-   200 Current estimation circuit-   201 Capacitor-   202 Sample and hold circuit-   203 Multiplier-   204 Voltage control current source-   205 Subtractor-   206 Divider-   207 Maximum value circuit-   300 Current detector circuit-   400, 400 b Discharge prevention circuit-   401, 401 b Operational amplifier-   401 a Voltage follower circuit-   402 Diode-   402 a, 402 b Switching circuit-   403 b, 404 b, 405 b, 406 b Resistor-   500, 500 a, 500 b Differentiating circuit-   501, 501 b, 505 Capacitor-   502, 502 b, 504 Resistor-   503, 503 b Operational amplifier

The invention claimed is:
 1. A current estimation circuit for estimatinga current flowing thorough an inductor in a switching power supplydevice that converts an alternating current or direct current inputvoltage into a direct current output voltage utilizing an energyaccumulation and releasing action of the inductor accompanying a turningon and off of a switching element, said current estimation circuitcomprising: current detection means for detecting a current flowingthrough the switching element, and outputting a corresponding signalvoltage; a capacitor that is charged by the signal voltage from thecurrent detection means; means for determining an amplification factorof a terminal voltage of the capacitor; instantaneous value detectionmeans for detecting an absolute value of an instantaneous value of theinput voltage and an instantaneous value of the direct current outputvoltage; means for determining a reduction factor of the terminalvoltage of the capacitor in an off period of the switching element,based on the amplification factor of the terminal voltage of thecapacitor, the absolute value of the instantaneous value of the inputvoltage, and the instantaneous value of the direct current outputvoltage; and discharging means for discharging the capacitor so that theterminal voltage of the capacitor decreases in accordance with thereduction factor in an off period of the switching element, wherein thecurrent flowing through the inductor is estimated from the terminalvoltage of the capacitor.
 2. The current estimation circuit according toclaim 1, wherein the current detection means includes a currenttransformer.
 3. The current estimation circuit according to claim 2,wherein the current transformer is an AC current transformer.
 4. Thecurrent estimation circuit according to claim 1, further comprising adischarge prevention circuit that prevents a discharge of current fromthe capacitor to the current detection means.
 5. The current estimationcircuit according to claim 4, wherein the discharge prevention circuitincludes a diode that prevents the discharge of current.
 6. The currentestimation circuit according to claim 4, wherein the dischargeprevention circuit includes a switching circuit that is turned on andoff at a timing at which the switching element is turned on and off. 7.The current estimation circuit according to claim 1, wherein the meansfor determining the amplification factor of the terminal voltage of thecapacitor comprises a differentiating circuit.
 8. The current estimationcircuit according to claim 7, wherein the differentiating circuit has aconfiguration that uses the capacitor as a differential determiningcomponent.
 9. The current estimation circuit according to claim 8,wherein the differentiating circuit comprises an operational amplifierhaving an inverting input terminal and a non-inverting input terminal,one end of the capacitor being connected to the inverting input terminaland a reference potential being input to the non-inverting inputterminal, the differentiating circuit further comprising a resistor thatis connected between the inverting input terminal and an output terminalof the operational amplifier.
 10. The current estimation circuitaccording to claim 1, further comprising voltage selection means forselecting whichever of the signal voltage from the current detectionmeans and the terminal voltage of the capacitor is the higher voltage asa voltage that estimates the current flowing through the inductor. 11.The current estimation circuit according to claim 1, further comprisingvoltage selection means for selecting whichever of a correspondingreference voltage when the signal voltage from the current detectionmeans is zero and the terminal voltage of the capacitor is the highervoltage as a voltage that estimates the current flowing through theinductor.
 12. The current estimation circuit according to claim 1,wherein the reduction factor of the terminal voltage of the capacitor isobtained by determining (+di/dt)·(vo−vin)/vin, where di/dt is theamplification factor of the terminal voltage of the capacitor, vin isthe absolute value of the instantaneous value of the input voltage, andvo is the instantaneous value of the direct current output voltage.